The engineering of disease resistance in crops is a major focus of plant biotechnology. One of the most promising approaches to this problem is to engineer defense reactions that are closely related to natural defense mechanisms such as hypersensitive cell death at infection sites, where the cells immediately surrounding an infection site die in order to prevent further spread of the pathogen (Strittmatter, Bio/Technology 13 (1995), 1085-1089). The controlled generation of highly localized necrotic lesions depends, however, on restricting any cytotoxic activity to the infection sites. This therefore requires promoters that are rapidly and locally responsive to pathogen attack but that also show negligible activity in uninfected tissues.
Initial attempts using large promoter fragments from pathogenesis-related genes such as prpl-1 have suffered from the disadvantage that it is difficult to isolate a promoter that is totally pathogen specific with substantially no activity in non-infected tissue (Strittmatter, 1995). It seems likely therefore that very few, if any, naturally occurring promoters will be suitable for this purpose.
Recent advances in the detailed study of defense related genes have identified a number of functionally defined cis-acting regulatory DNA elements within pathogen inducible promoters (Korfhage, The Plant Cell 6 (1994), 695-708, Raventos, Plant J. 7 (1995), 147-155, Rushton, EMBO J. 15 (1996), 5690-5700). A number of cis-acting elements that are necessary for the response to pathogens have been defined. These include Boxes P and L from the parsley PAL genes (Logemann, Proc. Natl. Acad. Sci. USA 92 (1995), 5905-5909), Boxes H and G from soybean PAL and 4CL (Loake, Proc. Natl. Acad. Sci. USA 89 (1992), 9230-9234), together with a number of less well defined elements. However, while it was shown for a number of such cis-acting elements that they are necessary for elicitor inducibility it was not known whether these elements are sufficient to direct pathogen-induced expression in plant cells and plants on their own. Recently, it has only been shown for the Box W1 from parsley (Rushton, EMBO J. 15 (1996), 5690-5700) and ERE from the maize Prms (Raventos, Plant J. 7 (1995), 147-155) that four copies of these elements alone are sufficient to direct elicitor responsive expression to some extent in transient gene expression assays. However, inducibility and background level of expression of the constructs investigated in Rushton, 1996 and Raventos, 1995 greatly varied and at best an about 10-fold induction of reporter gene expression was observed that may not be sufficient to supply the above-described biotechnological needs. Accordingly, it was unclear whether these or any other cis-acting elements may be useful to specifically suppress or confer local gene expression in plants upon pathogen infection.
Thus, the technical problem of the present invention is to provide promoters that are rapidly and locally responsive to pathogen attack but show negligible activity in uninfected parts of the plant and that can be used for engineering of disease resistant crops.
The solution to this technical problem is achieved by providing the embodiments characterized in the claims.